[Take 2] Unexpected duplicate join points in "Core" output?

Viktor Dukhovni ietf-dane at dukhovni.org
Sat Nov 20 01:02:49 UTC 2021


[ Sorry wrong version of attachment in previous message. ]

The below "Core" output from "ghc -O2" (9.2/8.10) for the attached
program shows seemingly rendundant join points:

      join {
        exit :: State# RealWorld -> (# State# RealWorld, () #)
        exit (ipv :: State# RealWorld) = jump $s$j ipv } in

      join {
        exit1 :: State# RealWorld -> (# State# RealWorld, () #)
        exit1 (ipv :: State# RealWorld) = jump $s$j ipv } in

that are identical in all but name.  These correspond to fallthrough
to the "otherwise" case in:

           ...
           | acc < q || (acc == q && d <= 5)
             -> loop (ptr `plusPtr` 1) (acc * 10 + d)
           | otherwise -> return Nothing

but it seems that the generated X86_64 code (also below) ultimately
consolidates these into a single target... Is that why it is harmless to
leave these duplicated in the generated "Core"?

[ Separately, in the generated machine code, it'd also be nice to avoid
  comparing the same "q" with the accumulator twice.  A single load and
  compare should I think be enough, as I'd expect the status flags to
  persist across the jump the second test.

  This happens to not be performance critical in my case, because most
  calls should satisfy the first test, but generally I think that 3-way
  "a < b", "a == b", "a > b" branches ideally avoid comparing twice... ]

======== Associated Core output

    -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
    main2 :: Addr#
    main2 = "12345678901234567890 junk"#

    -- RHS size: {terms: 129, types: 114, coercions: 0, joins: 6/8}
    main1 :: State# RealWorld -> (# State# RealWorld, () #)
    main1
      = \ (eta :: State# RealWorld) ->
          let {
            end :: Addr#
            end = plusAddr# main2 25# } in
          join {
            $s$j :: State# RealWorld -> (# State# RealWorld, () #)
            $s$j _ = hPutStr2 stdout $fShowMaybe4 True eta } in
          join {
            exit :: State# RealWorld -> (# State# RealWorld, () #)
            exit (ipv :: State# RealWorld) = jump $s$j ipv } in
          join {
            exit1 :: State# RealWorld -> (# State# RealWorld, () #)
            exit1 (ipv :: State# RealWorld) = jump $s$j ipv } in
          join {
            exit2
              :: Addr# -> Word# -> State# RealWorld -> (# State# RealWorld, () #)
            exit2 (ww :: Addr#) (ww1 :: Word#) (ipv :: State# RealWorld)
              = case eqAddr# ww main2 of {
                  __DEFAULT ->
                    hPutStr2
                      stdout
                      (++
                         $fShowMaybe1
                         (case $w$cshowsPrec3 11# (integerFromWord# ww1) [] of
                          { (# ww3, ww4 #) ->
                          : ww3 ww4
                          }))
                      True
                      eta;
                  1# -> jump $s$j ipv
                } } in
          joinrec {
            $wloop
              :: Addr# -> Word# -> State# RealWorld -> (# State# RealWorld, () #)
            $wloop (ww :: Addr#) (ww1 :: Word#) (w :: State# RealWorld)
              = join {
                  getDigit :: State# RealWorld -> (# State# RealWorld, () #)
                  getDigit (eta1 :: State# RealWorld)
                    = case eqAddr# ww end of {
                        __DEFAULT ->
                          case readWord8OffAddr# ww 0# eta1 of { (# ipv, ipv1 #) ->
                          let {
                            ipv2 :: Word#
                            ipv2 = minusWord# (word8ToWord# ipv1) 48## } in
                          case gtWord# ipv2 9## of {
                            __DEFAULT ->
                              case ltWord# ww1 1844674407370955161## of {
                                __DEFAULT ->
                                  case ww1 of {
                                    __DEFAULT -> jump exit ipv;
                                    1844674407370955161## ->
                                      case leWord# ipv2 5## of {
                                        __DEFAULT -> jump exit1 ipv;
                                        1# ->
                                          jump $wloop
                                            (plusAddr# ww 1#)
                                            (plusWord# 18446744073709551610## ipv2)
                                            ipv
                                      }
                                  };
                                1# ->
                                  jump $wloop
                                    (plusAddr# ww 1#) (plusWord# (timesWord# ww1 10##) ipv2) ipv
                              };
                            1# -> jump exit2 ww ww1 ipv
                          }
                          };
                        1# -> jump exit2 ww ww1 eta1
                      } } in
                jump getDigit w; } in
          jump $wloop main2 0## realWorld#

======== Executable disassembly

The jumps at "-1->" and "-2->" that correspond that "otherwise" have the
same target.  The duplicate "load+cmp" with "q" is at "-3->" and "-4->":

    0000000000408de8 <Main_main1_info>:
      408de8:       48 8d 45 e8             lea    -0x18(%rbp),%rax
      408dec:       4c 39 f8                cmp    %r15,%rax
      408def:       0f 82 c8 00 00 00       jb     408ebd <Main_main1_info+0xd5>
      408df5:       b8 79 dd 77 00          mov    $0x77dd79,%eax
      408dfa:       31 db                   xor    %ebx,%ebx
      408dfc:       b9 60 dd 77 00          mov    $0x77dd60,%ecx
      408e01:       48 39 c1                cmp    %rax,%rcx
      408e04:       74 66                   je     408e6c <Main_main1_info+0x84>
      408e06:       0f b6 11                movzbl (%rcx),%edx
      408e09:       48 83 c2 d0             add    $0xffffffffffffffd0,%rdx
      408e0d:       48 83 fa 09             cmp    $0x9,%rdx
      408e11:       77 59                   ja     408e6c <Main_main1_info+0x84>
-3->  408e13:       48 be 99 99 99 99 99    mov    $0x1999999999999999,%rsi
      408e1a:       99 99 19
      408e1d:       48 39 f3                cmp    %rsi,%rbx
      408e20:       73 0c                   jae    408e2e <Main_main1_info+0x46>
      408e22:       48 6b db 0a             imul   $0xa,%rbx,%rbx
      408e26:       48 01 d3                add    %rdx,%rbx
      408e29:       48 ff c1                inc    %rcx
      408e2c:       eb d3                   jmp    408e01 <Main_main1_info+0x19>
-4->  408e2e:       48 be 99 99 99 99 99    mov    $0x1999999999999999,%rsi
      408e35:       99 99 19
      408e38:       48 39 f3                cmp    %rsi,%rbx
-1->  408e3b:       75 49                   jne    408e86 <Main_main1_info+0x9e>
      408e3d:       48 83 fa 05             cmp    $0x5,%rdx
-2->  408e41:       77 43                   ja     408e86 <Main_main1_info+0x9e>
      408e43:       48 8d 5a fa             lea    -0x6(%rdx),%rbx
      408e47:       48 ff c1                inc    %rcx
      408e4a:       eb b5                   jmp    408e01 <Main_main1_info+0x19>
      408e4c:       0f 1f 40 00             nopl   0x0(%rax)
      408e50:       c2 00 00                retq   $0x0

-- 
    Viktor.
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